This invention relates to copper powder for electroconductive paints, more particularly to a copper powder for electroconductive paints (hereinafter referred to as conductive paint(s) which has improved storage stability and environmental resistance and improves the adhesion of a paint film in which it is used without reduction of the conductivity of the copper powder and its shielding effect against electromagnetic waves. The invention relates also to a process for preparing the copper powder.
One of the electromagnetic wave shielding materials known heretofore for the purpose of protecting electronic equipments from disturbance or jamming by electromagnetic waves is a conductive paint in which a conductive filler such as nickel powder, silver powder, copper powder, or carbon powder is kneaded with any of various binder resins such as thermoplastic resins. Shielding against electromagnetic waves is accomplished by applying the paint on the surface of molded articles by spraying or brushing. Conductive paints of copper type are inexpensive in comparison with those of silver or nickel types and have excellent shielding effect.
However, in the conductive paints of copper type, a uniform dispersion state cannot be realized because the copper particles tend to agglomerate together in the paint, which leads to inferior storage stability. Moreover, the conductive paints of copper type tend to be easily oxidized by environmental factors such as heat and humidity, which easily cause deterioration of environmental resistance and conductivity (attenuation of shielding effect) of the copper powder. Many methods have hitherto been proposed for the purpose of solving these problems. The methods include the preparation of conductive paints by blending a binder resin and an organic titanate into copper powder (Japanese Patent Laid-Open Application No. 36553/81), the surface treatment of copper powder with a coupling agent (Japanese Patent Laid-Open Application No. 30200/85), coating of the particles of electrolytic copper powder with an organic titanate (Japanese Patent Laid-Open Application No. 174661/84), coating of the particles of copper powder with an organic aluminum (Japanese Patent Laid-Open Application No. 179671/84) and the like.
The conductive paints thus obtained with copper powder are improved to a certain extent in storage stability and environmental resistance without a decrease in the conductivity of the copper powder and its shielding effect against electromagnetic waves.
However, conductive paints obtained with copper powder in the prior art, when used by the application thereof onto a substrate or matrix, are inferior in adhesion to the substrate and do not always exhibit excellent storage and environmental resistance.
This invention has been achieved in view of the above described background. An object of this invention is to overcome the problems accompanying conventional copper powders for conductive paints and to provide a copper powder for conductive paints which has improved storage stability and environmental resistance as well as the effect of imparting good adhesion to the paint film without reduction of the conductivity of the copper powder and shielding effect with respect to electromagnetic waves. Another object is to provide a process for preparing the copper powder according to the invention.